Achilles tendinopathies affect the health of much of the population of the United States. With improved understanding of the tendon repair process, better treatment strategies could lead to better outcomes for Achilles tendon healing. The overall aims of this study are to delineate which progenitor cells are recruited to initiate Achilles tendon repair and which matrix components are essential to the organization of collagen fibrils during the healing process. In this proposal, it is hypothesized that during Achilles tendon repair progenitors are recruited, differentiate into tenocytes, and begin forming matrix in a manner similar yet not definitively identical to progenitor cell commitment matrix formation which occurs during tendon development. It is hypothesized that progenitors within the Achilles tendon exist and have the capacity to initiate an intrinsic repair mechanism. It is also hypothesized that small leucine-rich proteoglycans (SLRPs), matrix molecules found within the tendon niche, are essential to collagen fibrillogenesis and remodeling during repair. Specifically, it is hypothesized that Class II SLRP fibromodulin is essential to collagen fibrillogenesis during repair. In this training project, progenitor cells of the Achilles tendon will be isolated and characterized in mouse tissue through molecular biological and immunohistochemical techniques; moreover, progenitors in and around the tendon will be localized by labeling of proliferating cells in transgenic (scleraxis promoter/green fluorescent protein, ScxGFP) mice to allow for characterization of intrinsic and/or extrinsic repair candidate cells. Then it will be determined which cell population(s) is/are responsible for suitable healing responses at the common site of rupture in the Achilles tendon via punch lesion injury model with transgenic ScxGFP mice (Aim 2). Moreover, the roles of SLRPs in collagen fibrillogenesis during repair will examined throughout the punch lesion model (Aim 3.1), and specifically the healing response of Achilles tendon injuries will be examined in fibromodulin-deficient mice (Aim 3.2). Studies proposed here are designed to determine the reparative role of each cell population in and around the Achilles tendon during the proliferative phase of tendon repair. Moreover, through this injury model, elucidation of the involvement of SLRPs in tendon healing is anticipated. Improved understanding of the involvement of progenitors and SLRPs within their niche could lead to improved therapeutic strategies for bolstered healing or prevention of further degenerative changes caused by overuse.